Neurotrophins are polypeptides that play a role in the development, function, and/or survival of certain cells, including neurons, oligodendrocytes, Schwann cells, hair follicle cells, and other cells. The death or dysfunction of neurons and other cell types has been directly implicated in a number of neurodegenerative disorders. It has been suggested that alterations in neurotrophin localization, expression levels of neurotrophins, and/or expression levels of the receptors that bind neurotrophins are therefore linked to neuronal degeneration. Degeneration occurs in the neurodegenerative disorders Alzheimer's, Parkinson's and ALS, among others. Degeneration of oligodendrocytes can occur in central nervous system injury, multiple sclerosis, and other pathological states.
A variety of neurotrophins have been identified, including Nerve Growth Factor (NGF), Neurotrophin-3 (NT-3), Neurotrophin-4/5 (NT-4/5), Neurotrophin 6 (NT-6) and Brain Derived Neurotrophic Factor (BDNF). Neurotrophins are found in both precursor form, known as pro-neurotrophins, and in mature form. The mature forms are proteins of about 120 amino acids in length that exist in physiological states as stable, non-covalent approximately 25 kDa homodimers. Each neurotrophin monomer includes three solvent-exposed β-hairpin loops, referred to as loops 1, 2, and 4 that exhibit relatively high degrees of amino acid conservation across the neurotrophin family.
Mature neurotrophins bind preferentially to the receptors Trk and p75NTR, while pro-neurotrophins, which contain an N-terminal domain proteolytically removed in mature forms, interact principally with the p75NTR receptor and through their N-terminal domains, with the sorting receptor sortilin (Fahnestock, M., Michalski, B., Xu, B., Coughlin, M.D. (2001) Mol Cell Neurosci 18, 210-220; Harrington, A. W. et al. (2004) Proc Natl Acad Sci USA 101, 6226-6230; Nykjaer. A. et al., (2004) Nature 427, 843-848). The p75NTR receptor interacts with Trks and modulates Trk signaling, but is also independently coupled to several signaling systems, including pro-survival signals, IRAK/TRAF6/NFκB, PI3/AKT, and pro-apoptotic signals, NRAGE/JNK (Mamidipudi, V., Li. X., Wooten, M. W. (2002) J Biol Chem 277, 28010-28018; Roux, P. P., Bhakar, A. L., Kennedy, T. E., Barker, P. A. (2001) J Biol Chem 276, 23097-23104; Salehi, A. H., et al. (2000) Neuron 27, 279-288).
When administered for therapeutic use, neurotrophins exhibit suboptimal pharmacological properties, including poor stability with low serum half lives, likely poor oral bioavailability, and restricted central nervous system penetration (Podulso, J. F., Curran, G. L. (1996) Brain Res Mol Brain Res 36, 280-286; Saltzman, W. M., Mak, M. W., Mahoney, M. J., Duenas, E. T., Cleland, J. L. (1999) Pharm Res 16, 232-240; Partridge, W. M. (2002) Adv Exp Med Bio 513, 397-430). Additionally, the highly pleiotropic effects of neurotrophins achieved through action of the dual receptor signaling network increases the chances of adverse effects.
It has been suggested that the unliganded form of p75NTR is proapoptotic, and that homodimerization induced by neurotrophin binding eliminates the effect (Wang, J. J., Rabizadeh, S., Tasinato, A., Sperandio, S., Ye, X., Green, M., Assa-Munt, N., Spencer, D., and Bredesen, D. E. (2000) J Neurosci Res 60, 587-593), consistent with studies showing no effects on survival of monomeric p75NTR ligands, including monovalent Fabs (Maliartchouk, S., Debeir, T., and Beglova, N. Cuello, A. C., Gehring, K, and Saragovi, H. U. (2000) J Biol Chem 275, 9946-9956) and monomeric cyclic peptides (Longo, F. M., Manthorpe, M., Xie, Y. M., Varon, S. (1997) J Neurosci Res 48, 1-17), while related bivalent forms in each study promote cell survival. However, these monomeric ligands may not engage the receptor in the same way as the natural ligands. Though active NGF is a homodimers containing 2 potential p75NTR binding sites, recent structural evidence suggests that it engages only one p75NTR molecule, disallowing the binding of another (He, X. L., Garcia, K. C. (2004) Science 304, 870-875).
Unfortunately, technical and ethical considerations have thus far hampered the development of therapeutic agents based upon neurotrophins. For example, it is technically difficult to produce sufficient quantities of pure neurotrophins using recombinant DNA techniques. Additionally, although it is possible to utilize human fetal cells to produce neurotrophins, the ethical ramifications raised by the use of such cells (typically obtained from an aborted fetus) have all but prevented the utilization of this approach. Accordingly, there is an unmet need in the art for the development of small molecule agents with favorable drug-like features based upon neurotrophins that are capable of targeting specific neurotrophin receptors for use in the treatment of disorders or diseases.